In optical fiber technology, wavelength division multiplexed (WDM) couplers are used to combine or separate optical signals having different wavelengths. As the WDM couplers are being more broadly applied in the telecommunications, data communications and CATV industries, the fiber optic component industry is now confronted with increasing requirements for WDM couplers with higher level of performance and reliability as well as lower cost
The performance, reliability and cost of the WDM couplers depend heavily on their design and packaging technologies. Currently, two major kinds of design and packaging technologies are being widely employed in manufacturing the WDM couplers and each kind has its own advantages and disadvantages. In applying a first kind of technology for designing and packaging the WDM couplers, all optical parts are bonded together by applying epoxy bonding. The applications of this first type of WDM couplers show potential reliability risk of epoxy bonding in long-term operation.
FIG. 1A shows the structure of a typical WDM coupler manufactured according to the first kind of design and packaging technology based on epoxy bonding The WDM coupler includes a dual fiber pigtail 25, a GRIN lens 35, a WDM filter 40, a GRIN lens 50, and a single fiber pigtail 60. In a typical manufacturing process, the GRIN lens 35, the WDM filter 40 and the GRIN lens 50 are first fixed together by applying a heat-curing epoxy 45. The relative position of the GRIN lens 35 to the fiber pigtail 25 is adjusted to achieve a lowest transmission loss from the input fiber 15 to the output fiber 20 for optical signals having reflection wavelengths. Then the dual fiber pigtail 25 is fixed to the GRIN lens 35 by applying a heat-curing epoxy 30. Then the relative position of the GRIN lens 50 to the fiber pigtail 60 is adjusted to achieve a lowest transmission loss from the input fiber 15 to the output fiber 65 for optical signals having transmission wavelengths. And then, the single fiber pigtail 60 is fixed to the GRIN lens 50 by applying a heat-curing epoxy 55. The conventional method and system provides the WDM couplers with good performance and reliability suitable for many types of applications. However, the WDM couplers manufactured according to the conventional method and system have a risk of failure when they are applied in high power optical transmission systems. In general, the heat-curing epoxies inevitably spread over all the optical paths in the WDM couplers. More specifically, the heat-curing epoxies 30, 45 and 55 spread over the optical paths between the dual fiber pigtail 25 and the GRIN lens 35, between the GRIN lenses 35, 50 and the WDM filter 40 and between the GRIN lens 50 and the single fiber pigtail 60, respectively. Under long-term operation, the epoxies 30, 45 and 55 when exposed to the transmitted optical signals may gradually become degraded and susceptible to damages and thus lead to unreliable performance after continuously absorbing the optical signal energy. In the typical WDM coupler, the diameter of the optical signal beam is changing from about 10 .mu.m at the epoxy 30 to about 450 .mu.m at the epoxy 45 to about 10 .mu.m at the epoxy 55. Thus, the optical signal power densities at the epoxies 30 and 55 are about 2500 times higher than that at the epoxy 45. Therefore, the risk for high optical power damage is significantly higher at the epoxies 30 and 55 than at the epoxy 45. The difficulties are specially pronounced for transmission of optical signals of high power. Because of the heat absorption problem, many optical system designers and operators now prefer or even demand to have all optical paths of the WDM couplers epoxy-free. Due to the significantly high power density and thus reliability risk, as the first step toward all epoxy-free optical paths, the optical system designers and operators now require not to use any epoxy on the optical paths between the GRIN lenses and the fiber pigtails. However, by applying the conventional WDM method and system, this epoxy-free optical path requirement can not be easily achieved. Thus, further development of reliable fiber optic components with high level of performance and reliability is limited by these difficulties.
In a pending patent application as shown by FIG. 1B, entitled "Low-Cost Wavelength Division Multiplexed Coupler With Flexible and Precise Optical Path Adjustment", filed recently by the present inventor, improvements are achieved for both the long-term high-power operation reliability and cost of the WDM couplers. In the pending application, while epoxies are prevented to spread over or diffused into the optical paths between the GRIN lenses 110', 160' and the fiber pigtails 135', 175' by employing several holding tubes 120', 130', 165' and 180'. Also, low-cost standard single fiber pigtails are used to replace expensive special single fiber pigtails. As a result, while the optical paths between the GRIN lenses and the fiber pigtails are epoxy-free, the costs of the WDM couplers are also reduced. The improved WDM couplers have significantly reduced risk of high optical power damage and lower cost Therefore, the improved WDM couplers can be employed in fiber optic components for broader scope of applications that is much less limited by the reliability and cost problems of the WDM couplers as that encountered in the prior art However, in the pending application, there is still epoxy diffused onto the optical path between the WDM filter 105' and the GRIN lens 110'. Therefore, there is still risk that in the long-term high-power operation, the epoxy 115' may gradually become degraded and susceptible to damages and thus leads to unreliable performance after continuously absorbing the optical signal energy. Thus, further development of the WDM couplers with high level of reliability is limited by these difficulties.
Therefore, a need still exists in the art of design and manufacturing of the WDM couplers to provide improved material compositions, device structure, and manufacturing processes to overcome the difficulties discussed above. Specifically, a technique to provide the WDM couplers with all optical paths being epoxy free is required. Since production costs have been being an important factor prohibiting practical implementation of fiber optic technology, it is also highly desirable that the cost of such a technology would be as low as possible.